Analyze the method of power field effect tube protection circuit design

Analyze the method of power field effect tube protection circuit design

Do you know how to analyze the design of power FET protection circuit? What does it do? What is a protection circuit?

Do you know how to analyze the design of power FET protection circuit? What does it do? What is a protection circuit?

In view of the fact that there are some unstable factors in the power circuit, the circuit designed to prevent such unstable factors from affecting the circuit effect is called a protection circuit. For example, there are overcurrent protection, overvoltage protection, overheating protection, no-load protection, short circuit protection, etc.

Analyze the method of power field effect tube protection circuit design

Design of Power Field Effect Tube Protection Circuit

Power MOSFETs have many advantages, but MOSFETs have a relatively fragile ability to withstand short-term overloads, especially in high-frequency applications. Therefore, in the application of power MOSFETs, a reasonable protection circuit must be designed to improve the reliability of the device. sex. The power MOSFET protection circuit mainly has the following aspects:

1) Prevent the gate di/dt from being too high:

Due to the driver chip, the output impedance is low. Directly driving the power tube will cause the driven power tube to turn on and off quickly, which may cause Voltage oscillation between the drain and source of the power tube, or may cause the power tube to suffer excessively high Di/dt caused by misconduction. In order to avoid the occurrence of the above phenomenon, a resistor is usually connected in series between the output of the MOS driver and the gate of the MOS tube, and the size of the resistor is generally selected to be several tens of ohms.

2) Prevent overvoltage between the gate and source. Because the impedance between the gate and the source is very high, the sudden change of the voltage between the drain and the source will be coupled to the gate through the capacitance between the electrodes, which will generate a relatively high gate-source spike voltage. It will cause the breakdown of the very thin gate-source oxide layer. At the same time, the gate can easily accumulate charge and the gate-source oxide layer will break down. Therefore, the zener tube must be connected in parallel with the MOS tube gate to limit the gate voltage in the zener tube. Below the voltage value, the MOS tube is protected from breakdown. The parallel resistance of the MOS tube gate is to release the gate charge and prevent the charge from accumulating.

3) Protection against overvoltage between drain and source

Although the drain-source breakdown voltage VDS is generally very large, if the drain-source is not equipped with a protection circuit, it is also possible that the drain spike voltage will be generated due to the sudden change of the device switching instantaneous current, which will damage the MOS tube, and the faster the power tube will switch. , The higher the overvoltage generated. In order to prevent device damage, protection measures such as Zener diode clamp and RC snubber circuit are usually adopted.

When the current is too large or a short circuit occurs, the current between the drain and source of the power MOSFET will increase rapidly and exceed the rated value. The power MOSFET must be turned off within the time specified by the overcurrent limit value, otherwise the device will be burned out Therefore, a current sampling protection circuit is added to the main loop. When the current reaches a certain value, the drive circuit is turned off by the protection circuit to protect the MOSFET. Figure 1 is the protection circuit of the MOSFET, which can clearly see the function of the protection circuit.

Lithium battery protection circuit

It consists of two field effect transistors and a dedicated protection integrated block S–8232. The overcharge control tube FET2 and the overdischarge control tube FET1 are connected in series in the circuit, and the battery voltage is monitored and controlled by the protection IC. When the battery voltage rises to 4.2V, The overcharge protection tube FET2 is turned off and charging is stopped. In order to prevent malfunction, a delay capacitor is generally added to the external circuit. When the battery is in a discharging state, when the battery voltage drops to 2.55V, the over-discharge control tube FET1 is cut off and stops supplying power to the load.

Overcurrent protection is to control FET1 to cut off and stop discharging to the load when a large current flows through the load. The purpose is to protect the battery and the field effect tube. Overcurrent detection uses the on-resistance of the field effect tube as the detection resistance, monitors its voltage drop, and stops discharging when the voltage drop exceeds the set value. A delay circuit is generally added to the circuit to distinguish between surge current and short-circuit current. The circuit has complete functions and reliable performance, but it is highly professional, and the dedicated integrated block is not easy to buy, and it is not easy to imitate by amateurs.

Because overcharge or overdischarge of Li+ batteries may cause an explosion and cause personal injury, safety is the main concern when using this type of battery. Therefore, commercial lithium-ion battery packs usually include protection circuits like DS2720 (Figure 7). DS2720 provides all the protection functions required for rechargeable Li+ batteries, such as: protecting the battery during charging, preventing circuit overcurrent, and extending battery life by limiting the battery’s discharge voltage.

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